PH is King. It is a must. I don't know how anyone gets by without keeping it in check. I was going to say maybe my bro but even he can't make it to the finish line because he won't PH. Starts good and falls apart.

At first it looks just like what we went through with thrips. But you say no bugs.
Yes you need more light but I think between PH and heat you don't stand a chance. Get those under control and see if they come around.